Endothelial barrier integrity is a critical requirement for physiological regulation of lung fluid balance. Following inflammatory lung vascular injury that compromises the endothelial barrier, endothelial regeneration and barrier restoration are requisite for survival. The molecular mechanisms that regulate these processes remain poorly understood, hence there currently is no efficacious option to prevent or treat persistently leaky lung microvessels, the hallmark of human acute lung injury. Our long term goal using molecular, cellular, and in vivo approaches is to elucidate the mechanisms of endothelial regeneration and barrier repair following lung vascular injury and identify novel therapeutic targets to prevent or reverse lung vascular injury and leaky microvessels. Our studies from the first cycle of this grant have demonstrated the essential role of FoxM1 in regulating endothelial regeneration and re-annealing of endothelial cell-cell contacts and thereby the recovery of endothelial barrier integrity following inflammatory lung vascular injury. Our Supporting Data presented here show that the endothelial-specific loss of the hypoxia-inducible factor HIF-1 results in complete inhibition of FoxM1 induction in the pulmonary vasculature and defective endothelial barrier repair in response to inflammatory injury. Thus, we will test the hypothesis that stabilization of HIF-1 is required for endothelial barrier repair through transcriptional regulation of FoxM1 expression following lung vascular injury. The proposed studies address the Specific Aims described below. In AIM #1, we will determine the central role of endothelial expression of HIF-1 in mediating endothelial barrier repair following sepsis-induced lung vascular injury. In AIM #2, we will delineate signaling mechanisms of HIF-1-mediated endothelial barrier repair. We will address the role of FoxM1 as the key effector of HIF-1. In AIM #3, we will address the role of HIF-1 stabilization by inhibiting the oxygen sensor prolyl hydroxylases (PHDs) in activating lung endothelial barrier repair. We will also address the potential clinical relevance of our findings in animal models to the pathogenesis of ARDS in patients. With the data from these comprehensive studies, we will delineate the fundamental mechanisms of activation of the intrinsic program for endothelial regeneration and barrier repair following lung vascular injury. We seek to identify novel therapeutic approaches to target leaky microvessels for the prevention and treatment of acute lung injury and its severe form, the acute respiratory distress syndrome.